(a) Field of the Invention
The present invention relates to a variable-gain circuit and, more particularly, to a variable-gain circuit for use in an amplifier.
(b) Description of the Related Art
In a wireless communication system, the transmitted wave attenuates in an amount depending on the transmission distance. Especially, in a mobile communication such as a cellular phone system wherein the transmission distance differs depending on the location of the mobile station, a variable-gain circuit is generally used for correcting the attenuation. (or attenuated amount) corresponding to the transmission distance.
FIG. 1 shows transmission/reception blocks of a conventional mobile station, wherein each of a low-noise amplifier 103, an intermediate frequency amplifier 105, a driver amplifier 111 and the like includes a variable-gain circuit. Two of typical conventional variable-gain circuits will be exemplified for description.
FIG. 2 shows one of the conventional variable-gain circuits, which is described in JP-A-6-164249. A common emitter transistor 1 has a collector connected to a power supply source 10 via a load 2, through which an amplified output 4 is fed. The base of transistor 1 is connected to a base bias circuit 5 including resistors 51 and 52, and to a variable current source 6. The variable current source 6 includes a current source transistor 61 and a serial resistor 64 connected to the collector of transistor 61.
In the conventional variable-gain circuit of FIG. 2, the amplifying gain of the common emitter transistor 1 is varied by varying the base bias current of the variable current source 6. More specifically, the collector current of transistor 61 is varied by controlling the base input of transistor 61, whereby the base bias current of transistor 1 is varied to vary the gain of the common emitter transistor 1.
FIG. 3 shows another of the conventional variable-gain circuits, which is described in JP-A-10-260734. A common emitter transistor 1 has a collector connected to a power supply source 10 via a load 2, through which an amplified output 4 is fed. The base of transistor 1 is connected to a base bias circuit 5 including resistors 51 and 52, and to a variable-impedance circuit 8 via a DC-blocking capacitor 9. The variable-impedance circuit 8 includes a common base transistor 81 and an emitter resistor 84 thereof.
In the conventional variable-gain circuit of FIG. 3, the variable-impedance circuit 8 controls the shunt current of the input signal shunted toward the ground for obtaining the variable gain. The base of transistor 81 is grounded in terms of AC sense (or as viewed with respect to an AC signal), and thus the control of the base input voltage of transistor 81 varies the base-emitter impedance of transistor 81, whereby the shunt current of the input signal is controlled to vary the gain.
It is desired in the variable-gain circuit for designing the control voltage generator with ease that the gain curve, which is obtained by plotting the decibel gain of the variable-gain circuit the control voltage, be exactly a linear curve. However, the gain curve is actually not linear in the conventional variable-gain circuit. The conventional variable-gain circuit of FIG. 2 has a gain curve such as shown in FIG. 4, which is convex toward the top of the graph, whereas the conventional variable-gain circuit of FIG. 3 has a gain curve such as shown in FIG. 5, which is convex toward the bottom of the graph.
In view of the above problems in the conventional variable-gain circuits, it is an object of the present invention to provide a variable-gain circuit having an improved gain curve.
It is another object of the present invention to provide a variable-gain circuit capable of allowing an amplifier including the variable-gain circuit to have an improved amplification characteristic.
The present invention provides a variable-gain TO circuit including a functional circuit for receiving an input signal to output an output signal having a gain with respect the input signal, a first gain control section for receiving a first control signal to control the functional circuit to operate based on a first gain curve of decibel gain plotted in a graph against the first control signal, the first gain curve being convex toward a bottom of the graph, a second gain control section for receiving a second control signal to control the functional circuit to operate based on a second gain curve of decibel gain plotted in a graph against the second control voltage, the second gain curve being convex toward a top of the graph, the first and second control signals being applied so that said first gain curve and said second gain curve cancel each other in non-linearity.
In accordance of the variable-gain circuit of the present invention, since the first gain control section controls the functional circuit to have the gain curve which is convex toward the bottom of the graph and the second gain control section controls the functional circuit to have the gain curve which is convex toward the top of the graph, application of the first control signal to the first gain control section and the second control signal to the second control section allows the first gain curve and the second gain curve to cancel each other in the non-linearity, thereby improving the overall gain curve toward the linearity.